Adding qiime map file

This adds the creation of the qiime mapping file (combination of prep/sample template) at the when the prep template is added. Note that the code is ready for review, it tests pass, but we should merge https://github.com/biocore/qiita/pull/677 first. 
I think you don't need the `tolist()` call you can just do `set(st.sample_name)`.